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Aeromobility and climate: it's complicated!

posted 21 Mar 2022, 11:31 by Barry McMullin   [ updated 21 Mar 2022, 11:32 ]

[Participant contribution to the online workshop Academic aeromobility in a post-pandemic future organised by the Tyndall Centre, Manchester, 22 March 2022. This is a lightly edited version of an email response to a colleague who, reviewing the possibilities for “flying less”, asked me to “… sketch out the issues re aviation emissions compared to other transport forms or to point me to some literature that would be helpful.” What follows is my fairly unstructured brain dump…]

Flying is certainly a very greenhouse-gas emissions-intensive form of transport. Nonetheless it is complicated to compare it with other transport options with any precision. For many reasons:

  • Point-to-point flying will use the shortest possible route, whereas surface travel for the same journey will generally be longer. Of course, if a direct flight is not available, then the multi-hop flight distance for a particular trip may be longer than surface.

  • The majority of fuel consumption in most powered transport is to move the “vehicle” (airplane, train, bus, car…) rather than the passengers. So the “emissions per passenger” depends critically on the “occupancy” (relative to the carriage capacity of the vehicle). But this varies hugely from case to case. So that makes deciding on emissions benefits for different travel modes on a “case by case” basis very complex and difficult. (Mind you: the question of higher “class” air travel does significantly skew the occupancy question: obviously a given aircraft can only carry fewer business class, or much fewer first class, passengers, so the “emissions per passenger” go up in fairly exact proportion.)

  • Direct comparison is further complicated by the fact that flying doesn’t just release greenhouse gases, especially CO2 (like all fossil fuel burning, including fossil-fueled surface vehicles), but it does so at high altitude. There is good science suggesting that this makes the climate impact (per tonne or litre of fuel burned) significantly greater for flying than surface travel - perhaps as much as a factor of two or more.

  • It can be argued (and is by many people) that “the airplane will be flying anyway”, so the emissions associated with having one more or less passenger is negligible. Or putting that the other way around, by choosing to, say, drive a car (especially single occupancy) for a given trip, instead of flying, total emissions “associated” with the trip (car + passenger, since the plane flies “anyway”) may actually be higher rather than lower. So it’s actually better to fly than not (if that keeps occupancy factors on planes high)! But against that, of course, one can argue that it’s the aggregation of trips that ultimately matters (Anderson’s “systems level”). So individual decisions not to fly, if done consistently and at scale, would, by sheer economics, ultimately result in fewer actual planes being scheduled to fly, and thus consistently lower emissions. Though, on its own, that’s still a weakish argument: if all those people still travel as far and as often, but now, in, say, single occupancy cars, then the total emissions are likely substantially higher.

  • There again, that was comparing flying with car travel, on the presumption that the car wouldn’t otherwise travel. But that won’t apply if comparing flying to travel on some other “shared” service (bus, train, ferry) where the other vehicle would also be travelling anyway.

  • An alternative way of coming at this then is to assume that we are working with some fixed total amount of passenger-km, and asking whether it makes sense to try to minimise the proportion of that undertaken by flying, as a long term, “system” objective? Viewed that way, a critical consideration is that, for surface travel (including high speed rail) there are existing, more-or-less mature, technical options to use non-fossil energy to power this (specifically, electricity from non-fossil fuel sources - solar, wind, hydro, nuclear…). For the moment only a minority of surface travel is actually powered in this way: but at least there is the prospect of progressively decarbonising the electricity source in this case. But there is little serious prospect of decarbonising flying, for at least several decades into the future (albeit, I am discounting aviation lobby arguments here, which rest on use of biofuels and/or “offsetting”). That being the case, one can say there is a strong argument that, as long as flying is fossil fueled, it just has to be abandoned. (Maybe, sometime in the future, we’ll have serious non-fossil-fuel options for flying, and maybe then flying can come back into use: but right now, full decarbonisation of energy logically requires that we stop flying.)

  • OK, so that was all premised on a certain “fixed” amount of travel (total passenger-km) and asking whether some or all of it should be switched away from flying. But perhaps the “core” issue here is not “flying” per se, but the total amount of (fossil fueled) travel by any means: the total passenger-km undertaken. While emissions will vary somewhat (in complex and sometimes counter-intuitive ways) according to modal choices, the biggest, most reliable, gain would be by reducing total (fossil-fuel-powered) travel by all modes.

  • But even then, we still come back to flying as a special case. It is precisely because it is so fast, that it enables, facilitates, and encourages much more total passenger-km than would be likely to be undertaken if only slower modes were available. OK, high speed rail might be subject to a similar criticism, but then we come back to the fact that, at least in principle, rail can be very largely decarbonised whereas, as yet, flying can’t. So further growth in high speed rail travel is not inextricably tied to growth in emissions in the way that air travel is.

  • Global inequity (in consumption and in climate change responsibility) is implicit in everything above: but I think it’s fair to say that flying raises the equity question in a particularly acute form: the sheer pointed insult of rich people flying over poor and vulnerable people, blithly contributing further to their threats and risks, is so grotesque that to contemplate it seriously seems almost unbearable (so I, like most of the global “jet set”, just don’t contemplate it at all).

  • And taking all those things together, we just have the sheer symbolism, at least when it comes to individuals, but especially organisations, that claim to be working to fight climate change. While the best thing for us to do is minimise our absolute consumption impact (including passenger-km) overall, flying is such an extravagantly intensive form of climate pollution that engaging in it just can’t but undermine the organisational message.

As I said, it’s complicated!

Some additional references:

Finally, to everyone who happens to chance on this post, good luck pursuing flying less measures in your own local organisational context, whatever that may be!